FYI - The Apera ph600S has a replaceable probe tip as well, and it also calculates the slope of the probe when you want to check it and make sure you're still in the acceptable range of accuracy.
From my understanding, it is time to replace any probe or unit when the offset falls outside the +/- 25 mV or the slope of the unit falls outside 85- 105 mEv. It's a bit of a headscratcher, but if you read it several times, you begin to understand the values of slope and offset. Way too much information now, but it is worthwhile understanding the concept since at some point a pH meter is no longer reliable and providing accurate numbers that we base meat safety on. If your unit doesn't provide the mV numbers in some manner then you have no idea if your numbers are correct after a year of use or so.
Here's an excerpt from
https://www.sigmaaldrich.com/conten...h/General_Information/1/labwarenotes_v1_6.pdf
..........That is, when the meter is calibrated it is done to compensate for the changes that have taken place to the electrode since the previous calibration. There are many influences, including aging, temperature, coatings and chemicals used, that will affect the probe characteristic. So the question becomes, what is typical and how do you check it? A pH meter is a very sensitive voltmeter. When a pH probe is placed into a solution, an mV potential is generated in response to the hydrogen ion concentration [H+]. The theoretical voltage generated can be determined by the Next equation. Theoretically at 25 degrees C, a pH 7.0 solution will generate 0 mV and there will be a 59.16 mV change for each pH unit. So at pH 4.0, +177.48 mV will be generated while at pH 10.0, with a lower H+ concentration relative to pH 7.0, a potential of -177.48 mV will be generated. Again, this is theoretical and does not represent the real world behavior. In fact, a new pH electrode will generate between +/- 10 mV in pH 7.0 and will have a slope percentage between 95 and 105%. The slope percentage is determined by dividing the actual voltage generated by the theoretical and then multiplied by 100. The following two examples show electrodes with different characteristics, where the voltage generated was measured with a pH meter.
Example 1: An electrode in pH 7.0 buffer generates +15 mV. When placed in pH 4.0 buffer it generates +175 mV. The net difference between the two buffers is +160 mV, which is then divided by +177.48 mV. The result, 0.901, is then multiplied by 100 to give a slope percentage of 90.1%. Example 2: An electrode in pH 7.0 buffer generated -45 mV while in pH 4.0 it generated +115 mV. The net difference is 160 mV and, as seen from example 1, equates to a 90.1% slope.
Example 2 is a prime example of how a probe can be calibrated but not be operating with an acceptable probe characteristic. That is, a slope of 90% is acceptable but the offset of -45mV is not. An offset shift can be the result of a build-up of dirt on the probe. This will affect the response time. Also, if the coating comes off with use then the characteristic will change and the calibration will no longer be valid. Typically, the offset should fall within +/-25 mV and the slope should be between 85 and 105%.
(live long and prosper)
